A) Field of the Invention
The present invention relates to a liquid crystal display.
B) Description of the Related Art
A vehicle mount information display apparatus is desired to have a very low display luminance of a background display area and a dark display area in order to enhance expensive looking.
A display apparatus with a fluorescent display tube widely used conventionally has the disadvantages that glass substrates used for the display apparatus are thick and heavy, and in addition there is the problem that a special driving power source is used.
A liquid crystal display apparatus is light in weight and can use a vehicle mount power source as a driving power source. However, a conventional liquid crystal display apparatus has an insufficient contrast in front viewing and right/left viewing.
In this specification, a liquid crystal display apparatus is intended to mean a display apparatus constituted of a liquid crystal display for information display, a backlight equipped with an emission source, a driver circuit and a control circuit for controlling the operations of these components.
A normally black type liquid crystal display has been developed recently which uses an inorganic light emitting diode (LED) as a light source of a backlight to emit light generally at a single wavelength and increase a contrast dramatically only at this wavelength, and is used in a vehicle mount information display apparatus.
For a liquid crystal display capable of realizing good normally black display without depending upon an emission wavelength of a backlight, the structure is known in which a vertical alignment mode (VA mode) liquid crystal cell is disposed between approximately crossed-Nichol disposed polarizers. In the vertical alignment mode (VA mode), liquid crystal molecules in a liquid crystal layer formed between two upper and lower glass substrates are vertically or approximately vertically aligned relative to a substrate surface. As a liquid crystal display having the above-described structure is observed along a glass substrate normal direction, the optical characteristics are almost equal to the optical characteristics of crossed-Nichol disposed two polarizers. Namely, since an optical transmissivity becomes very low, it is possible to realize high contrast display relatively easily.
An invention of a liquid crystal display is disclosed (for example, refer to Japanese Patent No. 2047880) in which a viewing angle compensator (C plate) having negative uniaxial optical anisotropy or a viewing angle compensator (negative biaxial film) having negative biaxial optical anisotropy is inserted at one or both positions between the upper polarizer and upper glass substrate and between the lower polarizer and lower glass substrate. Even if the liquid crystal display is observed along an oblique direction, it is possible to suppress a rise in optical transmissivity and a fall in contrast so that good display is realized.
For this viewing angle compensation method, effective conditions have been proposed (for example, refer to Japanese Patent No. 3330574) for an in-plane retardation and an arrangement of an in-plane slow axis respectively of a negative biaxial film.
An invention of a liquid crystal display is disclosed (for example, refer to Japanese Patent No. 3299190) in which good viewing angle characteristics are obtained by using a combination of a wavelength plate of approximately a half wavelength having biaxial optical anisotropy, and a C plate. However, the liquid crystal display described in Japanese Patent No. 3299190 requires a constituent element of the wavelength plate of approximately a half wavelength for realizing a retardation of approximately a half wavelength in order to realize a retardation of approximately a half wavelength independently from an observation direction. It is therefore necessary in practical use to provide positive biaxial optical anisotropy so that it is difficult to realize this liquid crystal display.
An invention of a liquid crystal display is also well known (for example, refer to Japanese Patent No. 3863446) using a combination of a negative biaxial optical anisotropy and a C plate is also well known, instead of using the combination of the wavelength plate of approximately a half wavelength having biaxial optical anisotropy and the C plate as in the invention described in Japanese Patent No. 3299190. Japanese Patent No. 3863446 describes that an in-plane retardation of the biaxial film is 190 nm or smaller and a retardation of a liquid crystal layer of the liquid crystal cell used is 200 nm to 500 nm.
In order to acquire good viewing angle characteristics even during the application of voltage, multi domain alignment is effective in which liquid crystal molecules are oriented along a plurality of directions in one pixel. In order to realize this in a VA mode liquid crystal display, there are known an oblique electric field alignment control method (for example, refer to Japanese Patent No. 3834304) of devising the shape of an electrode to generate an electric field in a liquid crystal layer and control the alignment of liquid crystal molecules along the oblique electric field direction, and a liquid crystal molecule alignment control method (for example, refer to Japanese Patent No. 2947350) of controlling alignment by forming bank-shaped projections on the substrate surface.
If importance is attached to the viewing angle characteristics of a liquid crystal display in the right/left direction, it is possible to obtain good viewing angle characteristics by using a liquid crystal cell not of multi domain alignment but of mono domain alignment in which liquid crystal molecules are aligned in a uniform direction in the whole liquid crystal cell plane. It is possible to realize uniform mono domain alignment, for example, by a photo alignment method (for example, refer to Japanese Patent No 2872628) for a vertical alignment film and a rubbing method (for example, refer to Japanese Patent Laid-open Publication No. 2005-234254) for a vertical alignment film having special surface free energy.
If a VA mode liquid crystal display is multiplex driven at a duty of ¼ to 1/240, a retardation Δnd of the liquid crystal layer is required to be larger than at least 320 nm, more preferably larger than 360 nm. Unless sharpness of the electro optical characteristics is made as good as possible, it becomes difficult to maintain both the high contrast characteristics and a high optical transmittance during an on-voltage state in high duty driving, both being characteristic to the VA mode during high duty driving.
An optical film presently used for a liquid crystal display is a primary film manufactured in such a manner that raw resin is formed into a continuous film by a melting cast method or a melting extrude method and the continuous film is eventually wound around a roll. In order to develop a retardation in the primary film in an in-plane direction and a thickness direction, mainly a drawing process is executed. The drawing process is a process of drawing a primary film in a heated state along a roll winding direction (MD direction) and a direction TD (direction perpendicular to the MD direction) by a roll-to-roll method.
Many of resin films distributed in markets as negative biaxial films are formed by biaxially drawing a primary film having a thickness of about 0.2 mm or thinner and made of norbornene based cyclic olefin polymer (COP) along the MD and TD directions to develop an in-plane slow axis along the MD or TD direction.
An in-plane retardation Re (defined by Re=(nx−ny)×d where nx is an in-plane refractive index of a resin film in a lag phase axis direction, ny is an in-plane refractive index in an advance phase axis direction, and d is a film thickness) is larger than 0 nm and equal to or smaller than 300 nm, preferably larger than 4 nm and equal to or smaller than 300 nm, or more preferably larger than 30 nm and equal to or smaller than 300 nm. A retardation Rth in a thickness direction (defined by Rth=((nx+ny)/2−nz)×d where nz is a refractive index in the thickness direction) is equal to or smaller than 350 nm. If an Nz factor (defined by Nz=(nx−nz)/(nx−ny)) used as a ratio between a refractive index in the in-plane direction to that in the thickness direction does not satisfy the condition that the factor is larger than 1 and smaller than 56, more preferably larger than 1 and smaller than 12, it can be considered that realizing in-plane uniformity of Re and Rth is difficult.
A thickness of a film after the drawing process is about several tens μm. Material of a base film of a polarizer having essentially the optical characteristics of a C plate and a protective film has been improved by using, as a base, triacethyl cellulose (TAC) used by these films. This material is subjected to the drawing process to develop an in-plane slow axis along the MD or TD direction, and negative biaxial optical films formed in this manner are also sold in markets. An in-plane retardation Re of this film has a narrower range than that of norbornene based COP, and is about 40 to 70 nm. A retardation Rth in the thickness direction is 120 nm or lager to 220 nm or smaller.
If the invention described in Japanese Patent No. 3330574 is reduced in practice by disposing a commercially available negative biaxial film only between the liquid crystal cell and one polarizer, a retardation Δnd of the liquid crystal layer is smaller than about 500 nm. If a negative biaxial film is disposed between the liquid crystal cell and both the polarizers, a retardation Δnd of the liquid crystal layer is smaller than about 850 nm.
However, as will be described later with reference to comparative examples, if the liquid crystal layer of a liquid crystal display having the structure that a viewing angle compensator is disposed between the liquid crystal cell and both polarizers has a large retardation Δnd, there appears the phenomenon that display is hardly visually recognized along the right/left direction during bright display at a large polar observation angle, particularly at an angle larger than 45°. Therefore, in order to obtain good display quality under the condition that a retardation Δnd is large, i.e., under the driving condition of a large duty ratio, it is considered effective to use a method of disposing a biaxial film between the liquid crystal cell and one polarizer or a method of laminating a negative biaxial film and a C plate as described in Japanese Patent No. 3863446.
However, the former method is effective only for a retardation Δnd to about 500 nm and it is also difficult to realize good display characteristics during high duty driving. The latter method requires an expensive C plate so that a manufacture cost of a liquid crystal display is raised.